180
DOC.
14
PROOF
OF
AMPERE'S
CURRENTS
703
wore
a
very
light
mirror made
from
a
silvered
microscope
covering
glass.
The
light
of
a single
wire
lamp was
thrown
on
the
mirror
through
the
space
between the two
coils.
The
reflected
rays
formed
an
image
on a
scale
placed
at
a
distance of
45
cm
When the
cylinder
was
set
vibrating
this
image
was
broadened
into
a
band,
the width of
which
determined the
double deviation.
In order
to
obtain
resonance,
it must
of
course
be
possible
to
regulate
the
length
of
the
glass
wire. For
this
purpose we
used
a
clamping arrangement
by
which the
glass
wire
could
be
tightly
held at different
points
of its
length.
The
clamp
and
the
suspending
wire
with the
cylinder
could rotate
together
about
a
vertical axis
in
a
fixed
column. The
effective current
was
read
on a precision
instrument.
Finally,
the whole
apparatus
was
surrounded
by
an arrangement by
which the terrestrial
magnetic
field could be
compensated.
We shall
revert
to
it
further
on.
$
5. The
experiments.
Let
us now
examine the
principal disturbing causes.
1.
At the ends of the
cylinder alternating
poles are
induced.
Acting
on
these
the horizontal
component
of the terrestrial
field
can
give
rise to
a
couple alternating
with the
same
frequency as
the
current and
tending to
rotate the
cylinder
about
a
horizontal
axis.
(Effect
I).
Rotations
of
this
kind have
not, however, been observed
by us.
2.
According
to
the views of Weiss the
ferromagnetic crystals
[11] are lying irregularly
in
all
directions. It
may
therefore
happen
that
some
of them
are
directed in
such
a
way
that their
magnetism
is
not
reversed
by
the
alternating
field. In this
case
there
will be
a
permanent
horizontal
component
of the
magnetisation,
which,
acted
upon
by
the
alternating
horizontal
component
of
the
magnetic
field
in the
coil,
will
give
rise
to
an alternating
couple
around the
vertical axis
with the
same frequency
and
phase as
the
alternating
field
(Effect
II).
3.
The
axis about
which
the
cylinder
rotates
will not
coincide
accurately
with
its
magnetic
axis.
A
permanent
horizontal
magnetic
force such
as
that of terrestrial
magnetism,
will therefore
produce
torsional
oscillations of the
cylinder.
The
couple
which excites these
oscillations
has
the
same
phase
as
the
magnetisation
and
(in
the
case
of
strong currents)
as
the alter-
nating
current
itself.
4.
It
is
easily
seen
that
the
Foucault
currents which
are
induced